1. Field of the Invention
This invention pertains to x-ray phosphors. More particularly it relates to cerium-activated yttrium gadolinium phosphors.
2. Prior Art
Calcium tungstate has been used as a phosphor for x-ray intensifying screens for many years. Until recently no other x-ray phosphor has gained acceptance. Within the past two years, however, various terbium activated rare earth oxysulfides have been used. These new phosphors while brighter than calcium tungstate yield a green emission and they necessitate to achieve the maximum benefit, a green sensitive film. The film most prevalent in usage for x-ray usage is blue sensitive film.
Cerium-activated yttrium phosphate and cerium-activated gadolinium phosphate and the mixtures thereof are disposed as photoluminescent and cathodoluminescent materials by Ropp in the Journal of Electrochemical Society, Vol. 115, No. 5, 1968. Cerium-activated yttrium-phosphate was disclosed as being much brighter than cerium-activated gadolinium phosphate when excited by 3020 angstroms, 2910 angstroms, and 2610 angstroms wavelengths of excitation media. Cerium-activated gadolinium phosphate is disclosed as being about 10% as efficient as cerium-activated yttrium phosphate. All mixtures of the two materials have less brightness than cerium-activated yttrium phosphate. Furthermore, the materials were prepared by precipitating the phosphates from an aqueous solution. Such a process would not yield materials that have a sufficient particle size to enable their usage for x-ray phosphors. Furthermore, there is no disclosure in the article of the materials being excited by x-rays. It is well known in the art that all materials which are excited by the foregoing wavelengths are not necessarily luminescent when exposed to x-ray excitation media. It has been a desire on the part of the x-ray phosphor industry to achieve a material which is equivalent to calcium tungstate and would have a brightness approaching that of the rare earth oxysulfides to enable the reduction of the exposure to x-rays. Furthermore, most of the calcium tungstate which is used as an x-ray phosphor is manufactured outside of the continental limits of the United States.
It is believed therefore that it would be an advancement in the art to provide a material which yields an emission in the desired blue wavelength and is at least equivalent to calcium tungstate.